This is why the act of playing music by a human is called a "performance", an "interpretation" in some languages. The musician is not just playing a score exactly as it is written, the musician gives a part of his/her into the act and make the music his/her own to make art.
- the instrument
- the room or hall
- their personal experiences
- their personal philosophy of music (e.g., Gould and his anti-hedonistic philosophy)
- cultural norms and explicit respect paid thereof (e.g., in the interpretation of a Chopin mazurka)
- available recording technology and expertise
- available audio engineering technology and expertise
- what emotion they want to invoke or evoke to contextualize the music
- their patience
- their willingness to experiment
- etc.
It is not at all farfetched that a performer is taking most of these into account, directly or indirectly, to render a live or recorded performance.
It's not as if there's just this tradition of unwritten rules about interpreting a score that music students learn from their teachers, rather, it's that an interpretation is an amalgamation of experience, context, and on-the-spot reactive decision-making.
For an AI to successfully achieve this, they'll need the same variety of inputs and contexts to learn from.
The truth is that classical music fan are very very picky about what they like.
Computers have been "better" musician than human for a long time: a midi file is always played "perfectly". Yet it's sounds horrible, mechanised to our ears. The controlled imperfection of a real human musician is what makes the music beautiful.
I don't see any computer composing anything worth listening to in the foreseeable future (and I'm bullish on generative ai).